Twisting

Twisting happens when the bar does not stay parallel to your feet. One end of the bar gets farther away from your body and loads you unevenly. This is very dangerous for your back.

There are two ways this happens.

The most common is when people use an over/under grip on the bar and fail to keep it close. Pay attention when you use the over/under grip and avoid this mistake (and potential injury) by keeping the bar CLOSE.

Another time this happens is when there is asymmetrical strength in the body. Your body will always take the path of least resistance, and it may twist the bar to load up your strong side. You can use the mirror, unilateral work, and a friend's eyes to identify strength/development asymmetries in your body. Fix them using unilateral (dumbbells, single leg work) exercises.

Gapping

This is the most common fault I see in beginners and failed deadlifts.

Gapping occurs when the bar separates from the body. In a great deadlift the bar will maintain contact with your body the entire way up and down the lift. This is the reason long socks are a requirement for powerlifting competitions--it keeps competitor's flesh out of the knurling.

Apply force to the bar swiftly but gradually. You want to increase force on the bar until it lifts off the ground. Don't try to yank it off the floor.

Hitching

Hitching occurs when the bar stalls above the knee. People will then set the bar on their thighs to rest muscles for an instant before trying to jerk the bar up the rest of the way.

CrossFit has been made famous for this as it is legal in competition. Powerlifting federations do not allow hitching. In fact, powerlifting competitions will disqualify a deadlift if it stops moving up at any time even for an instant, with or without hitching.

Hitching is no good because it means you broke form at the beginning of your lift. You rounded your back and straightened your legs instead of extending your hips to get the bar off the ground.